The present invention concerns timepieces having a stepping motor and more particularly a control method and device for applying to the terminals of the winding of the stepping motor, a control signal comprising a series of drive pulses, each of the drive pulses itself being formed by a series of spaced elementary pulses.
The book entitled "Theory and Applications of Step Motors" by Benjamin C. Kuo, West Publishing Co., pages 173 to 180, proposes supplying the winding of a stepping motor, with a control signal of that type. In that prior art document, each of the drive pulses applied to the winding of the motor is cut into elementary pulses in the following manner: the voltage source used for feeding the motor is first connected to the terminals of the winding of the motor. The power supply source is disconnected from the winding and the winding is short-circuited as soon as the current flowing in the winding reaches a first predetermined value. The current in the winding then decreases and, when it reaches a second predetermined value, the power supply source is again connected to the terminals of the winding of the motor, the short-circuited condition of which is eliminated. Such a method permits the current flowing in the motor winding to be maintained at a substantially constant means value.
Nonetheless, if the voltage of the power source varies, the power supplied to the motor varies in the same manner so that the known method does not permit the power supplied to the motor in each drive pulse to be maintained at a constant level, when using a power supply source, the electromotive force and the internal resistance of which vary in the course of time.
British Pat. No. 2,006,995 proposes chopping each drive pulse which is applied to the winding of the motor, using two separate, predetermined values of the chopping rate, the higher value being used only when the motor is to provide an abnormally high force. For that purpose, the above-indicated British patent proposes using a means for detecting the load on the motor.
This known control apparatus also suffers from the disadvantage of not taking into account fluctuations in the voltage supplied by the power source, which are due to variations in the electromotive force and/or internal resistance of the power source.
Now, in electronic timepieces, there is a tendency at the present time to use a lithium-type battery as the electrical power supply source. It is known that the electromotive force produced by such batteries decreases relatively substantially during the service life of the battery, and that the internal resistance of the battery is subject to substantial variations during the life of the battery and under the effect of variations in temperature. The above-mentioned reduction in electromotive force and/or the variations in internal resistance may cause the motor to stop, so that the timepiece no longer works, well before the battery reaches the end of its service life. In order to overcome that disadvantage, the size of the motor must be such that it can continue to operate even when the battery is supplying its lowest level of electromotive force and is at its highest level of internal resistance. This results in over-consumption by the motor throughout the major part of the service life of the battery.
British patent application No. 2,054,916 proposes supplying the winding of a stepping motor with drive pulses which are each formed by a series of elementary pulses, the width of which is determined in dependence on the value of the voltage which is supplied by the power source when the latter is connected to the terminals of resistors of known values. In accordance with that art, substantially every millisecond, the range of values in which the power source voltage falls is determined, and a form of drive signal is selected, in consequence, from five predetermined forms of signal.
That arrangement is therefore concerned with discontinuous adjustment of the level of power of the drive pulses in dependence on the voltage of the electrical power supply source, and the result is substantial variations in the motor torque which may cause steps to be lost. In addition, as the control action is discontinuous, it does not provide for the energy of the drive pulses to be efficiently controlled in dependence on the load to be driven by the motor.